In the semiconductor manufacturing and assembly industry, it is sometimes necessary to visually inspect the surfaces of electronic components to ensure that there are no defects. Machines in the industry often use computerized vision systems for various purposes such as for monitoring processes and inspecting finished or intermediate product outputs from individual machines. In one instance, after molding to form a semiconductor package, it is prudent to inspect the surface of the package for cavities or unacceptable unevenness of the surface. In such instances, three-dimensional optical inspection systems would be most ideal because of the ability to detect depth within the optical system's field of vision.
Inspection of semiconductor interconnects is especially crucial to ensure defect-free assembly of devices on printed circuit boards and flex circuits. Before assembly of semiconductor devices, two-dimensional (2-D) inspection of electronic components can ascertain if bumps or leads are missing or out of alignment, while three-dimensional (3-D) inspection can confirm that an electronic component is of a proper size and shape. 3-D inspection can also detect distortions of the surface contours of the substrate as aforesaid.
Among the several non-contact, optical methods of obtaining 3-D images of objects, one involves the projection of a grating image onto the object under scrutiny. FIG. 1 is a schematic illustration of an opto-mechanical inspection system of the prior art using a phase shifting technique with grating projection. A sinusoidal diffusion grating 10 comprising multiple equidistant and parallel lines or bars is frequently used for this purpose. Collimated light 12a incident on the sinusoidal grating 10 emerges as a modulated beam 12b such that its intensity has a unidirectional, sinusoidal spatial profile 14. This intensity-modulated beam 12b is incident over an object 16 to be inspected. The reflected beam 18 has a distorted intensity profile 20, the distortion being the result of the height variation of the object 16. The grating 10 is typically moved along its plane incrementally through a specific distance each time relative to the object 16. The direction of the motion is generally perpendicular to the orientation of the grating lines. Between successive strokes, images of the grating lines projected on the object are captured by a CCD camera that is positioned to view the distorted intensity profile 20 of the reflected beam 18.
In general, the grating 10 is moved through small incremental distances, typically fractions of a millimeter with accuracy of the order of 1–2 microns. At each position of the grating 10, an image of the pattern of the lines formed on the surface is captured by the CCD camera and recorded. A combination of these patterns gives rise to a depth profile along the surface of the object 16 so that its surface contour can be determined. Vision algorithms based on intensity and phase variations between these images are used to compute the height profile of the object. With the demands of modern-day semiconductor manufacturing and assembly systems, the motion of the grating has to be executed speedily and the grating positioned precisely at required locations in order to get accurate depth measurements while maintaining a high throughput.
In order to achieve the said speed and accuracy, the movement of the grating should preferably be actuated by a mechanism that is highly precise. Prior art apparatus for displacement of optical components are flawed in this respect. For example, U.S. Patent Publication No. 2004/28333 for “Tunable Optical Filter” teaches the use of an actuation means including a threaded drive shaft whose thread has leading and trailing thread faces. Threaded nut regions resiliently engage the thread faces of the drive shaft, the threaded nut regions being in communication with a filter plate for moving the filter plate relative to a radiation beam in response to rotation of the drive shaft member relative to the threaded nut regions. The drive shaft member is connected to a stepper motor, d.c. motor or linear motor for controllably rotating the drive shaft. In another prior art example, U.S. Pat. No. 5,307,152 for a “Moiré Inspection System” discloses the mounting of a grating on a translation stage that comprises a precision motorized micrometer that is used to drive the translation stage.
These prior art examples use motors that basically convert rotary motion to linear motion to control translation of the grating and are insufficiently precise for higher performance requirements, especially as there are a number of disadvantages associated with their designs. For instance, for practical reasons, a rotating screw and an associated nut that is movable on the screw cannot be coupled too tightly together so as to allow one to move relative to the other. Therefore, gaps exist between corresponding threads of the screw and nut that can give rise to backlash and hysteresis problems, especially during fast motion involving a change of direction of motion. Furthermore, the gap often gives rise to an offset between rotary and corresponding linear motion, which retards its ability to execute quick and accurate motion.
Therefore, it would be desirable to employ a displacement mechanism for the optical grating that avoids some of the above problems with the said prior art mechanisms. Furthermore, it would also be desirable to introduce a frictionless and wear-free structural support for sliding displacement of the grating. Conventional supports utilize roller bearings that encounter wear while rolling over surfaces and they lack accuracy and repeatability because of friction from contact with the surfaces on which they slide. It would be advantageous to implement a new support mechanism that has higher accuracy and repeatability. Flexures are especially suited for these purposes due to the excellent inherent repeatability of their motion trajectory devoid of friction and wear.